Apparatus for altering the magnetic state of a permanent magnet

ABSTRACT

An apparatus ( 10 ) for altering the magnetic state of a permanent magnet comprises a coil inductor ( 26 ) for generating and applying an induced magnetic field to the permanent magnet. The coil inductor ( 26 ) is provided in circuit between two charge storage elements ( 20, 24 ). The apparatus also comprises a discharge control circuit ( 28 ) for transferring charge alternately in opposed directions between the storage elements ( 20, 24 ) though the coil inductor ( 26 ) to generate a series of alternating polarity magnetic field pulses ( 167, 191 ) of decreasing magnitude in the coil inductor ( 26 ) or to generate a single magnetic pulse of relatively high strength. The apparatus ( 10 ) can be operated as either a magnetising or demagnetising device and is capable of demagnetising a column of 200 or more magnets.

BACKGROUND OF THE INVENTION

This invention relates to an apparatus for altering the magnetic stateof a permanent magnet and particularly, though not exclusively, to amagnetiser or demagnetiser for use with highly permanently magneticRare-Earth Transition Metal magnets such as Nd—Fe—B or Sn—Co basedmagnets

FIELD OF THE INVENTION

Demagnetisation of ferromagnetic components is often necessary inindustry to facilitate handling or coating of the components. Inaddition, demagnetisation also prevents unwanted pick up of magneticdebris.

One way of achieving demagnetisation is to heat the components to atemperature above their ferromagnetic Curie temperature; on cooling backdown to below the Curie temperature, the permanent magnetism is lost.This is a costly, time-consuming process which is not suitable for manymaterials due to corrosion problems or for an assembly containingplastics material, for example.

Demagnetisation of a ferromagnetic component may also be achievedmagnetically by applying successively smaller opposing magnetic fieldsto the magnetised component so as to drive the component aroundsuccessively smaller magnetic hysteresis loops, until the component isdemagnetised. For materials which are only slightly permanentlymagnetic, such as mild steel, the magnetic demagnetisation may readilybe achieved by slowly withdrawing the component from the centre of amagnetic field generated by a mains driven A.C coil inductor.

For materials which are more permanently magnetic, such as hardersteels, and ferrite and Alnico permanent magnets, a single shot“ringing” capacitor discharge demagnetiser is used. capacitor dischargemagnetisers work by discharging a charged bank of capacitors through acoil inductor thereby producing a magnetic field which magnetises thecomponent. Conventional capacitor discharge demagnetisers work on asimilar principle, but the demagnetising circuit is designed such thaton discharge, a decaying resonance or ringing occurs, withelectromagnetic energy transferred successively between the coilinductor and the capacitor bank. This ringing phenomenon, combined withthe natural loss of energy associated with coil inductors, ensures thegeneration of a reversing magnetic field of decaying amplitude whichdemagnetises the component.

There are many difficulties with magnetically demagnetising the mostpermanently magnetic materials such as Rare-Earth Transitional Metalmagnets based on Nd—Fe—B or Sm—Co. Use of a single-shot ringingdemagnetisation circuit is not possible for these magnetic materialsbecause any such circuit would not ring with sufficient efficiency, thatis with a high enough Q-factor, at the high power levels required forthese materials. At present, the only way of magnetically demagnetisingRare-Earth Transition Metal permanent magnets is to apply about 20 ormore magnetic pulses of reversing sign and decreasing amplitude with aconventional capacitor discharge demagnetiser. After the discharge ofeach pulse, the operator has to wait for the capacitors to recharge upto the new level and has to reverse the connections to the demagnetisingcoil. This is a very time-consuming procedure and is not practicable inan industrial environment.

SUMMARY OF THE INVENTION

It is desired to overcome the above-mentioned problems and to provide anapparatus which is capable of altering the magnetic state of a permanentmagnet in an efficient, controllable and relatively quick manner.

According to one aspect of the present invention there is provided anapparatus for altering the magnetic state of a permanent magnet, saidapparatus comprising: a magnetic field inducing device for generatingand applying an induced magnetic field to said permanent magnet, saiddevice being provided in circuit between two charge storage elements;and means for transferring charge alternately in opposed directionsbetween said storage elements through said magnetic field inducingdevice to generate a series of alternating polarity magnetic fieldpulses of decreasing magnitude in said device.

Preferably the apparatus is arranged to demagnetise a column of Nd—Fe—Bpermanent magnets, for example 200 or more magnets, in a singleoperation. This can be achieved by the magnetic field inducing devicebeing a coil inductor which is long enough to accommodate the column ofmagnets. The uniform demagnetisation or a column of permanent magnets isconsiderably more difficult than the demagnetisation of a single magnet.This difficulty is due in part to the differences of the degree ofpermeability at the ends of the column as compared with the middle ofthe column. However, the present invention advantageously overcomesthese problems and permits the demagnetisation of relatively largenumbers of permanent magnets in a single operation.

Preferably the transferring means is also arranged to discharge chargestored in the storage elements into the magnetic field inducing deviceto generate a single magnetic field pulse of sufficient amplitude tomagnetise the magnet. The apparatus may also be arranged to connecttogether both of the storage elements to provide a single charge storagemeans which has a greater charge storage capacity than either of theindividual charge storage elements. In this way, the apparatus canadvantageously be arranged to carry out both magnetisation anddemagnetisation in a fast and efficient manner.

Preferably, the apparatus further comprises adjusting means forcomparing the amount of charge present in the storage elements with apredetermined set level and for adjusting the amount to be equivalent tothe set level between each charge transfer. The provision of adjustingmeans advantageously allows the charge received by a storage means to betopped up to a predetermined set level before the next charge transfer.Accordingly, the size of the decreasing envelope of magnetic pulses canbe accurately controlled and, in particular, the amplitude of step sizebetween successive magnetic pulses can be set by the operator. The stepsize is important because if it is too large, the magnet, will be leftwith an undesirable residual magnetism after the demagnetisationprocedure, and if the step size is too small, the demagnetisationprocedure will take too long and not provide an industrially practicalsolution.

The apparatus may be arranged to commence each operation for alteringthe magnetic state of the magnet from a different storage element tothat used in the previous operation. By alternating the starting storageelement, the working life of the storage elements is advantageouslymaximised.

The magnetic field inducing device may comprise a plurality ofindividual magnetic field inducing devices, such as coil inductors,which are arranged to be selectively connected into circuit after eachoperation for altering the magnetic state of the magnet. The provisionof several magnetic field inducing devices advantageously reduces thetime period between successive demagnetisation or magnetisationoperations which would otherwise be required for the magnetic fieldinducing device to cool down between operations.

According to another aspect of the present invention there is providedan apparatus for changing the magnetic state of a permanent magnet to adesired magnetic state, said apparatus comprising: means for charging afirst charge storage element to a predetermined level; means forgenerating a magnetic field pulse by discharging said first storageelement into a second storage element via a magnetic field inducingdevice; means for generating another magnetic field pulse of a differentpolarity and a different magnitude than that of said previous pulse bydischarging said second storage element into said first storage elementvia said magnetic field inducing device; said generating means beingarranged to be operated alternately to provide a series of alternatingpolarity magnetic field pulses of decreasing magnitude in said device.

According to another aspect of the present invention, there is providedan apparatus for demagnetizing a permanent magnet by spiralling itaround its hysteresis loop, said apparatus comprising: means forgenerating an electromagnetic field; first and second charge storagemeans connected together and to said field generating means fortransferring charge between each other via said field generating means;and control means for controlling the charging and discharging thereofso as, in use of the apparatus, to cause said field generating means togenerate an alternating polarity reducing magnetic field.

According to another aspect of the present invention there is providedin or for an apparatus for altering the magnetic state of a permanentmagnet by application thereto of a magnetic field of alternatingpolarity and decreasing strength, a control circuit for controlling thegeneration of said magnetic field by discharge of first and secondcharge storage means through a magnetic field generating means, saidcontrol circuit being adapted and arranged for transferring chargebetween said first and second charge storage means by way of saidmagnetic field generating means so as to subject a permanent magnetwithin the magnetic field of said magnetic field generating means to asequence of alternating polarity magnetic impulses of progressivelydecreasing strength appropriate to the demagnetisation of the permanentmagnet.

The present invention also extends to a method of altering the magneticstate of a permanent magnet, said method comprising: providing amagnetic field inducing device for generating and applying an inducedmagnetic field to said magnet said device being provided in circuitbetween two charge storage elements; transferring charge alternately inopposed directions between said storage elements through said device togenerate a series of alternating polarity magnetic field pulses ofdecreasing magnitude in said device.

According to another aspect of the present invention there is provided amethod of changing the magnetic state of a permanent magnet, said methodcomprising: charging a first charge storage element to a predeterminedlevel; discharging said first storage element into a second chargestorage element via a magnetic field inducing device to generate amagnetic field pulse; discharging said second storage element into saidfirst storage element via said magnetic field inducing device togenerate another magnetic field pulse of different magnitude anddifferent polarity than that of said previous magnetic pulse; andrepeating said discharging steps to generate a series of alternatingpolarity magnetic field pulses of decreasing magnitude in said deviceuntil said permanent magnet has reached a desired magnetic state.

The above and further features of the invention are set forth withparticularity in the appended claims and together with the advantagesthereof will become clearer from consideration of the following detaileddescription of an exemplary embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic block diagram of a capacitor dischargedemagnetiser embodying the present invention;

FIGS. 2A, 2B, 2C and 2D are detailed circuit diagrams of the capacitordischarge demagnetiser of FIG. 1, and fit together as shownschematically in FIG. 2;

FIG. 3 is a flow diagram showing how the capacitor dischargedemagnetiser operates; and

FIG. 4 is a timing diagram showing how demagnetiser of FIG. 1 produces aseries of alternating polarity magnetic field pulses of decreasingamplitude from each capacitor discharge.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1 there is shown a schematic block diagram of acapacitor discharge demagnetiser 10 embodying the present inventionwhich can demagnetise a plurality of Rare-Earth Transition Metalpermanent magnets in a single operation. The demagnetiser 10 is poweredfrom a 240 Volt AC mains supply 12 which feeds a low-power transformer14 and a high-power transformer 16. The high-power transformer steps upthe mains supply voltage from 240 Volts to 550 Volts, namely a voltagewhich is large enough to carry out the demagnetisation procedure. Thelow-power transformer 14 is used for generating a D.C. power supply anda circuit timing signal which is discussed in detail elsewhere.

Rectifier thyristor bridge A 18 rectifies the stepped up voltage fromthe high-power transformer 16 and supplies current to capacitor bank A20. The use of thyristors instead of diodes enables the charging ofcapacitor bank A 20 to be controlled by selective firing of thethyristors. Similarly, rectifier thyristor bridge B 22 is also poweredby the high-power transformer 16 and selectively supplies charge tocapacitor bank B 24.

It should be noted that in the present embodiment each capacitor bank20, 24 as shown in FIG. 1 actually comprises two smaller capacitor banksconnected together in series as shown in FIG. 2. For the sake ofconvenience, references made herein to either capacitor bank 20, 24should be taken to be to the appropriate two smaller capacitor banksconnected in series.

The capacitor banks 20, 24 are connected together via a demagnetisingcoil inductor 26 and a discharge control circuit 28. The demagnetisingcoil inductor 26 applies an induced magnetic field to the permanentmagnets (not shown) which are to be demagnetised, in dependence upon thesize and polarity of the current flowing through the coil inductor 26.The discharge control circuit 28 can be triggered to allow charge flowto flow from either one capacitor bank 20,24 to the other capacitor bank20, 24 through the coil inductor 26.

The charging voltages of each capacitor bank 20, 24 is continuallysensed by a voltage sensing circuit 30 and a voltage difference signalis sent to a phase control circuit 32. The phase control circuit 32determines the correct voltage level that each capacitor bank 20, 24should be at and sends this information to a logic control and timingcircuit 34. The phase control circuit 32 is also coupled to thelow-power transformer 14 and generates a full-wave rectified signalwhich is used as a phase clock signal, described in detail hereinafter.

The logic control and timing circuit 34, is coupled to and controls theoperation of each thyristor bridge 18, 22. In this way, the logiccontrol and timing circuit 34 can increase the amount of actual chargestored in the capacitor banks 20, 24 to a correct level. In addition,the logic control and timing circuit 34 triggers the discharge controlcircuit 28 at the appropriate time to transfer charge stored in onecapacitor bank 20, 24 to the coil inductor 26. The direction of chargeflow between the capacitor banks 20, 24 is also controlled by the logiccontrol and timing circuit 34.

A charge dump circuit 36 is also provided and is coupled to each of thecapacitor banks 20, 24. The charge dump circuit 36 acts as a safetydevice to dump charge from the capacitor banks 20, 24 in the event of aninterruption in the discharging cycle. In addition, as the demagnetiser10 is kept in a charged up state between demagnetisation operations, thedump circuit 36 also allows the capacitor banks 20, 24 to be safelydischarged when the demagnetiser 10 is to be turned off.

The above described circuit is arranged to demagnetise a column of fullyor partially magnetised Rare-Earth Transition Metal permanent magnetssuch as Nd—Fe—B or Sm—Co based magnets. The principle of demagnetisationis based upon applying a reversing magnetic field of decaying amplitudeto the magnets which forces the magnetic material around its hysteresisloop in successively decreasing magnetic cycles, i.e. in a spiral. Theway in which the demagnetiser 10 achieves this is described below.

When the demagnetiser 10 is switched on, capacitor bank A 20 is chargedup to a predetermined level via the thyristor bridge A 18. Theabove-mentioned column of magnets is then placed within the coilinductor 26. The charge control circuit 28 is fired and the charge inthe capacitor bank A 20 is discharged into the coil inductor 26 therebyinducing a magnetic field pulse of given size and direction. Thedemagnetiser 10 is designed so that it rings once into and partiallycharges capacitor bank B 24. The capacitor bank B 24 is then similarlydischarged producing a magnetic field pulse in the opposite direction inthe coil inductor 26, and partially charges up capacitor bank A 20again. Each discharge of a capacitor bank 20, 24 into the coil inductor26 decreases the amount of charge being passed between the capacitorbanks 20, 24 and also reduces the resultant magnetic field pulse beingapplied to the column of magnets. The successive discharging is repeatedto generate a series of alternating polarity magnetic field pulses ofdecaying magnitude, until the capacitor banks 20, 24 are completelydischarged. At this point the column of magnets will be demagnetised.

At each state of the above process, the capacitor bank 20, 24 havingbeen partially charged by the discharge of the other capacitor bank 20,24, may be charged up to a predetermined level. This is done by firstmeasuring the difference in voltages across the capacitor banks 20, 24using the voltage sensing circuit 30. The measured voltage isproportional to the amount of charge present in a given capacitor bank20, 24. The measured voltage difference is compared with thepredetermined voltage level by the phase control circuit 32 and if thevoltage across the charged capacitor bank 20, 24 needs to be increased,a control signal is sent to fire the appropriate thyristor bridge 18, 22to increase the charge stored in the corresponding capacitor bank 20,24. Once the measured voltage difference accords with the predeterminedvoltage level, the charged capacitor bank 20, 24 is ready fordischarging. By reducing the predetermined voltage stored in the phasecontrol circuit 32 by a given amount (step) at each discharge stage, theprecise decaying amplitude of the reversing magnetic field being appliedto the column of magnets can be controlled.

FIGS. 2A, 2B, 2C and 2D show in detail the electronic circuitconfiguration of the capacitor discharge demagnetiser 10. The main partof the circuit comprises the thyristor bridges 18, 22, the capacitorbanks 20, 24, the discharge control circuit 28 and the demagnetisingcoil inductor 26. Rectifier thyristor bridge A 18, includes fourthyristors 1A, 2A, 3A, 4A arranged in a standard rectifying bridgeconfiguration with a capacitor 38 provided across the bridge. Rectifierthyristor bridge B 22 also comprises four thyristors 5B, 6B, 7B, 8B inthe same configuration and has a capacitor 40 positioned across thebridge. Each of the thyristors 1A, 2A, 3A, 4A, SB, 6B, 7B, 8B isconnected to an associated low-power charging transformer 42 which, whenactivated, generates a trigger pulse for firing each thyristor 1A, 2A,3A, 4A, SB, 6B, 7B, 83. A snubber network 44 is provided between thethyristor bridges 18, 22 to suppress any high-frequency signals whichmight cause misfiring of the thyristors.

The thyristor bridges 18, 22 convert the A.C. mains power supply 12output from the transformer 16 into a constant charging current for thecapacitor banks 20, 24. Each capacitor bank 20, 24 comprises two sets inseries of 24 high-voltage electrolytic capacitors connected in parallelto provide a total of 80,000 μF of capacitance per bank. Thesecapacitors are selected to each to operate at 325 Volts and each has avoltage rating well in excess of this voltage value. The instantaneousvoltage across each capacitor bank 20, 24 is measured and the differencetherebetween is displayed by a digital voltmeter 46 which is connectedto a resistor network 48 across the capacitor banks 20, 24. The displayof the voltage difference provides an indication of how the process isprogressing and also indicates when the process has been completed.

The discharge control circuit 28 comprises a pair of thyristors 50arranged to allow charge to flow in opposite directions. However, at anyone time only one thyristor is operational and so, current is onlyallowed to pass between the capacitor banks 20, 24, in one selecteddirection for each charge transfer. Each thyristor 46 is coupled to alow-voltage discharge transistor 52 which when activated generates anappropriately sized and shaped trigger pulse to fire the thyristor 50.

The rest of the capacitor discharge demagnetiser 10 is essentiallydivided between four circuit boards namely, the voltage sensing board54, the phase control circuit board 56, the logic circuit board 58 andthe power supply board 60.

The voltage sensing board 54 provides the voltage sensing circuit 30.Voltages present across each capacitor bank 20, 24 are input viaresistors 62, 64 to a differential amplifier 66. The output signal ofthe differential amplifier 66 represents the voltage difference betweenthe capacitor banks 20, 24 and serves to indicate how much charge ispresent in each capacitor bank 20, 24. The output of the differentialamplifier 66 is converted into an absolute voltage value signal at 67 byrectifier amplifier 68. This voltage value signal is then passed to thephase control circuit 32 on the phase control circuit board 56.

The phase control circuit board 56 includes a zero-crossing pointcircuit 70 which monitors the output of the low-voltage transformer 14and generates a phase clock signal at 72 for synchronising all of theevents that occur in the operation of the demagnetiser 10. Inparticular, the phase clock signal is supplied to the logic circuitboard 58 for synchronising the discharge of the capacitor banks 20, 24.

The phase control circuit 32 determines the set level to which eachcapacitor bank 20, 24 is charged during the demagnetisation operationand how that level decreases with each capacitor discharge event.operator selection of the appropriate step size capacitor 74 determinesthe step size by which the set level is to be decreased during thedemagnetisation procedure. The set level is determined by potentiometer76 which is also under operator control. The step size capacitors 74 andthe set level potentiometer 76 are both connected to a diode pumpcircuit 78. The diode pump circuit 78 is arranged to extract a smallamount of charge from a main capacitor 80 and transfer the charge to theselected step size capacitor 74 each time the pump circuit 78 is fired.This has the effect of reducing the initial set level voltage at 82output from the diode pump circuit 78 in a series of constant voltagesteps until the output set level voltage at 82 is zero.

A ready signal at 84 for initiating the discharge of the chargedcapacitor bank 20, 24 is produced from the output of a comparator 86which compares the present absolute voltage value signal at 67 from thevoltage sensing board 54 with the output voltage at 82 of the diode pumpcircuit 78. When the absolute voltage signal at 67 reaches thepredetermined set level voltage at 82 the comparator 86 drives the readysignal at 84 into an active condition. The resistor/capacitor circuit 88provides a 1 mS delay in the activation of the ready signal at 84. Theoutput of the comparator 86 is also passed to a charge timing circuit 90which comprises a phase control capacitor 92, a constant currentcharging transistor 94, a control circuit 96 for the charging transistor94 and an output circuit 98 coupled to the charging transformers 42.

The charge timing circuit 90 is input with the phase clock signal at 72and outputs a pulsed control signal at 100 for repetitively firing thecharging transformers 42. The phase angle of the control signal at 100is varied in dependence upon charge stored in the phase controlcapacitor 92. The phase control capacitor 92 is charged from thecharging transistor 94, the base of which is in turn controlled by thecontrol circuit 96. The control circuit 96 includes potentiometer 102for setting the rate of rise of the capacitor bank charging,potentiometer 104 for setting the starting point of the capacitor bankcharging and a comparator 106 for comparing the voltages generated fromeach potentiometer 102, 104. The absolute voltage value signal at 67 isinput into the control circuit 96 to generate a voltage acrosspotentiometer 102.

The output of comparator 86 is an active low signal which acts todischarge the phase control capacitor 92 of the charge timing circuit.90. In addition, the phase control capacitor 92 is connected to thelogic control board 53 via an override control line 108 which acts todisable the charge timing circuit 90 when required. When the overridecontrol line 108 is activated, transistor 110 is turned off and theoutput at 100 floats high. This causes the charging transistor 42 toalso be disabled so they cannot be fired.

The logic control and timing circuit 34 on the logic control board 58generates timing signals for enabling the operation of the chargingtransformers 42 and for controlling the discharging transformers 52.Each of the charging transformers 42 is coupled to a respective drivertransistor 116 which can selectively enable operation of the chargingtransformers 42. Each driver transistor 116 is operated in opposition,namely when one is switched on, the other is switched off. The bases ofthe driver transistors 116 are coupled to respective outputs 118 of abistable circuit 120 which determines which thyristor bridge la, 22 isto be operational, i.e. which capacitor bank 20, 24 is to be charged up.The start up configuration of the bistable circuit 120 is determined byflip-flop 122 which is provided to alternate the capacitor bank 20, 24which is first to be discharged in a demagnetisation operation.Alternating the start up capacitor banks 20, 24 for each demagnetisationoperation advantageously extends the operational life of the capacitorbanks 20, 24.

Each of the discharging transformers 52 is controlled by the output 124of a respective timer 126. The timers 126 are each configured togenerate a timing pulse of 100 ms duration when appropriately triggered.A four input Nand gate 128 is provided on the trigger input 130 of eachtimer 126 such that four input signals must be at a high logic level totrigger one of the timers. The outputs 118 of the bistable circuitprovide one input signal for each timer 126. These inputs are providedto permit operation of one timer 126 at one moment in time andsimultaneously to prevent operation of the other timer 126. Thisselection ensures that discharging of the capacitor banks 20, 24 onlyoccurs in one direction at a time.

Another input to the Nand gates 128 is provided by the ready signal at84 which indicates when the voltage level on the capacitor banks 20, 24is at the predetermined set level for the next discharge. The phaseclock signal at 72 is also input to the Nand gates 130 to ensure thatthe discharge triggering is synchronised with the phase of the powersupply 12.

The last input to the Nand gates 128 is a demagnetisation operationenable signal 132 which is output from a timer 134. This signal 132 isprovided for turning off the discharge timers 126 at the end of ademagnetisation operation. The timer 134 is configured to have a userselectable time delay, typically of the order of 5 seconds, which is setby potentiometer 136 in combination with a timing capacitor 138.

The timer 134 is triggered by the depression of a push button 140 whichis provided for the user to press when a demagnetisation operation is tobe commenced. In use, once the timer 134 has been triggered, it isprevented from reaching the end of its timing period by the continualdischarging of the timing capacitor 138 by transistor 142. However, oncethe end of a demagnetisation operation has been reached, as signified bythe continuous presence of an active ready signal at 84, the transistor142 is turned off for long enough to allow the timing capacitor 138 tocharge up and allow the timer 134 to reach the end of its timing period.The disenabling of the demagnetisation operation enable signal 132 alsoresets the phase control circuit 32 ready for the next demagnetisationoperation.

Referring now to FIGS. 3 and 4, the steps involved in operating theabovedescribed demagnetiser will now be described. The demagnetisationoperation commences with the turning on of the demagnetiser 10 at 150.At this time, one of the capacitor banks 20, 24 is selected and ischarged by a constant current at 152 because voltage across thecapacitor banks 20, 24 has not reached the predetermined set level.Once, the predetermined set level has been reached at 154, the chargingis disabled and the delay of 1 mS is generated. At the end of the delay,the ready to discharge signal is activated at 156.

The demagnetiser 10 is now ready to commence a demagnetisation operationand the operator can place one or more permanent magnets to bedemagnetised into the coil inductor 26. The demagnetiser 10 remains incharged state at 158 until the pushbutton 140 is depressed by theoperator at 160. The demagnetisation operation commences by thetriggering of the timer 134 at 162 and waiting at 164 until thezero-crossing point is reached (determined by phase clock signal). Thencharged capacitor bank A 20 is discharged through the inductance coil 26into the capacitor bank B 24 at 166. The discharge at 166 has the effectof generating a magnetic pulse 167, the size of which is determined bythe amount of charge that is discharged into the coil inductor 26.

Once the capacitor bank A 20 has discharged most of its charge and thecapacitor bank B 24 has received all of the charge not used by the coilinductor 26, the charge on the capacitor bank B 24 is isolated at 168.The isolation prevents any leakage of the transferred charge back intothe coil inductor 26. Once a 100 mS delay has been completed at 170, thepolarity of the bistable circuit 120 is changed at 172, the isolation ofthe capacitor bank B 24 is released at 174 and the set level voltage 175is reduced by 1 step at 176.

The capacitor bank B 24 is charged by a constant current at 178 andchecks are made at 180 to establish whether the measured voltage acrossthe capacitor bank B 24 is equivalent to the new set level voltage 181.When the new set level voltage 181 is reached at 182, the charging ofthe capacitor bank B 24 is stopped and a 1 ms delay is generated at 184.A ready to discharge signal becomes available at 186. Discharging of thecapacitor bank B 24 at 190 does not occur until the zero-crossing pointhas been reached at 188.

Discharge of the capacitor bank B 24 is from a lower set level voltage181 than the original set level voltage 175 and accordingly, a magneticpulse 191 is generated which is of a smaller magnitude than the previouspulse 167. The charge received by the capacitor bank A 20 is isolated at192 and the voltage level across the capacitor bank A 20 is maintaineduntil the end of a 100 mS delay at 194. Then the set level voltage ischecked at 196 to determine whether it is set at zero volts. If the setlevel voltage has not yet reached zero volts at 198., steps 172 to 194are repeated namely, the charging up of the capacitor bank A 20 to a newpredetermined set level and the discharging of the charged capacitorbank A 20 into the other capacitor bank B 24. However, if the new setvoltage is equivalent to zero volts, then the ready to discharge signalwill be permanently active which signifies the end of thedemagnetisation operation. The operation waits at 200 until the timer134 has timed out and then continues at 202 with resetting the timer126, 134, clocking the flip-flop 122 and resetting the set level voltageat 82 to a start level. The demagnetiser 10 is then ready at 204 tocarry out another demagnetising operation on a new set of permanentmagnets and as mentioned previously, this next operation is to becommenced from the other capacitor bank, in this case capacitor bank B24. Accordingly steps 152 to 204 are repeated.

Each demagnetisation operation generates heat in the coil inductor 26and the coil inductor 26 has to be cooled to a predetermined temperaturebetween successive operations. A fan (not shown) is provided in thedemagnetiser 10 for air cooling the coil inductor 26. However, it cantake several minutes after the end of one operation before the coil hascooled sufficiently for the next operation. In another embodiment of thepresent invention, the single coil inductor can be replaced by aplurality, for example 5, coil inductors in parallel which canselectively be switched into circuit between the capacitor banks 20, 24.By switching in a different coil inductor 26 after each operation it isnot necessary to wait for the coil inductor 26 to cool and the timetaken for carrying out a series of demagnetisation operations issignificantly reduced. The switching between different coil inductorscan be effected either manually or automatically using relays.

It is also possible to replace the two capacitor banks 20, 24 of thedescribed embodiment with a single capacitor, each plate of thecapacitor being used as a charge storage means. The importantrequirements for the charge storage means are that they can withstandthe high voltages to which they are subjected and that they can storesufficient charge for carrying out the demagnetisation operation.

The above described embodiments are designed to carry outdemagnetisation. However, it is to be well understood that the inventionis not limited to demagnetisation and can readily be used to magnetise apermanent magnet. A magnetiser embodying the present invention would bevery similar to the previously described demagnetiser 10. However,rather than reducing the set level voltage by the step size between eachcapacitor bank discharge, the capacitor banks would be connectedtogether in parallel and both charged up to their maximum level. Thenboth capacitor banks 20, 24 would be discharged in the same directionthrough the coil inductor 26 in a single non-ringing shot. The resultantmagnetic field pulse would be of a sufficient strength to magnetise themagnet or column of magnets placed in the coil inductor 26. It can beseen that the demagnetiser 10 can readily be modified to provide bothmagnetisation and demagnetisation operations; the required operationbeing selected by the use of a simple switch.

As the predetermined maximum charge level is set by the operator, thepermanent magnet can be magnetised to any level along its hysteresisloop, namely partial nagnetisation of the permanent magnet can becarried out. Similarly, in the demagnetiser 10, by setting the correctend voltage of the operation, partial demagnetisation can also becarried out. In this regard the use of the words “magnetise” or“demagnetise” in the claims should be understood to mean a respectiveincrease or decrease in the permanent magnetism of the magnetic materialand not be limited to a totally magnetised or totally demagnetisedstate.

Having described the present invention with reference to exemplaryembodiments thereof, it is to be clearly understood that this is by wayof illustration and example only and is not to be considered by way oflimitation, the scope of the present invention being determined by theappended claims. For example, the main output thyristors 18 and 22 canadvantageously be arranged to be triggered by lesser rated thyristors toobviate any risk of premature firing of the main thyristors and enablethe capacitor banks 20 and 24 to be more completely discharged, thecathode of each lesser rated thyristor being connected to the gate ofthe respective main output thyristor. Furthermore, in order to renderthe described embodiment insensitive to differences in the power supplyfrequency in different countries so that one and the same apparatus canbe used without need for modification in, say, 50 Hz countries such asthe United Kingdom (GB) and in 60 Hz countries such as the United Statesof America (US), the capacitor charging circuits can be made time andvoltage dependent rather than simply being voltage dependent as in thedescribed embodiment. This can be achieved by inclusion of an additionalIC in the circuit to make the phase control both time and voltagerelated, rather than just voltage related, the additional IC ensuringthat the phase angle, which is time dependent, is set correctly so thatthe voltage-related part can operate correctly. Additionally, a furtherfront panel button or switch may be provided, together with simplecontrol circuitry, to allow single-discharge magnetizing operation ofthe described embodiment if required.

What is claimed is:
 1. An apparatus for altering the magnetic state of apermanent magnet, said apparatus comprising: a magnetic field inducingdevice for generating and applying an induced magnetic field to saidpermanent magnet, said device being provided in circuit between twocharge storage elements; and means for transferring charge alternatelyin opposed directions between said storage elements through saidmagnetic field inducing device to generate a series of alternatingpolarity magnetic field pulses of decreasing magnitude in said device.2. An apparatus according to claim 1, further comprising charging meansfor charging any of said charge storage elements to a predetermined setlevel.
 3. An apparatus according to claim 2, further comprisingdisabling means for disabling the operation of said charging meansduring said charge transfer.
 4. An apparatus according to claim 1,further comprising adjusting means for comparing the amount of chargepresent in said storage elements with a predetermined set level and foradjusting the amount to be equivalent to said set level between eachcharge transfer.
 5. An apparatus according to claim 4, wherein saidadjustment means comprises means for measuring the amount of chargestored in each of the storage elements between each charge transfer. 6.An apparatus according to claim 4, wherein said adjusting meanscomprises means for supplying charge to each of said storage elementsbefore each charge transfer between said storage elements.
 7. Anapparatus according to claim 6, wherein the adjusting means furthercomprises means for controlling the supplying means to increase theamount of charge stored in any one of the storage elements to saidpredetermined set level.
 8. An apparatus according to claim 4, whereinsaid adjusting means is arranged to decrease said predetermined setlevel by a selected step size between each charge transfer.
 9. Anapparatus as claimed in claim 1 wherein said apparatus is arranged tocommence each operation for altering the magnetic state of said magnetfrom a different storage element to that used in the previous operation.10. An apparatus according to claim 1, wherein the apparatus operatesfrom an AC mains power supply and further comprises means for detectingthe phase of the AC mains power supply, said phase detection means beingarranged to supply a phase synchronised timing signal to said chargetransferring means for phase synchronising said charge transfers.
 11. Anapparatus according to claim 1, wherein said magnetic field inducingdevice comprises a coil inductor within which can be placed one or morepermanent magnets whose magnetic state is to be altered.
 12. Anapparatus according to claim 1, wherein said charge storage elementseach comprise a plurality of high-voltage electrolytic capacitors. 13.An apparatus according to claim 1, wherein said charge transferringmeans is arranged to discharge most of the charge held in one of saidstorage elements and to transfer a significant amount of said chargeinto the other of said storage elements during each charge transfer. 14.An apparatus according to claim 1, wherein said charge transferringmeans comprises a thyristor circuit arranged to selectively control thedirection and timing of charge flow between said storage elements. 15.An apparatus according to claim 6, wherein said charge supplying meanscomprises a pair of current rectifying thyristor bridges, each thyristorbridge being associated with one of said storage elements and beingcoupled to an AC mains power supply for rectifying the current suppliedfrom said power supply.
 16. An apparatus according to claim 1, furthercomprising means for dumping charge from said storage elements, saidstorage dump means being coupled to each of said storage elements andbeing arranged to effect a complete discharge of both of said storageelements.
 17. An apparatus according to claim 1 wherein said magneticfield inducing device comprises a plurality of separate field inducingdevices which are arranged to be selectively coupled into circuit aftereach operation for altering the magnetic state of said magnet.
 18. Anapparatus according to claim 1, wherein selectively operable means arefurther provided for enabling the discharge of charge stored in saidstorage elements into said magnetic field inducing device to generate asingle magnetic field pulse of sufficient amplitude to magnetise amagnet subject thereto.
 19. An apparatus according to claim 18, wherein,for magnetising a magnet, said apparatus is arranged to connect togetherboth of said storage elements to provide a single charge storage meanswhich has a greater charge storage capacity than either of saidindividual charge storage elements.
 20. A method of altering themagnetic state of a permanent magnet, said method comprising: providinga magnetic field inducing device for generating and applying an inducedmagnetic field to said magnet said device being provided in circuitbetween two charge storage elements; transferring charge alternately inopposed directions between said storage elements through said device togenerate a series of alternating polarity magnetic field pulses ofdecreasing magnitude in said device.
 21. A method of changing themagnetic state of a permanent magnet, said method comprising: charging afirst charge storage element to a predetermined level; discharging saidfirst storage element into a second charge storage element via amagnetic field inducing device to generate a magnetic field pulse;discharging said second storage element into said first storage elementvia said magnetic field inducing device to generate another magneticfield pulse of different magnitude and different polarity than that ofsaid previous magnetic pulse; and repeating said discharging steps togenerate a series ot alternating polarity magnetic field pulses ofdecreasing magnitude in said device until said permanent magnet hasreached a desired magnetic state.
 22. An apparatus for changing themagnetic state of a permanent magnet to a desired magnetic state, saidapparatus comprising: means for charging a first charge storage elementto a predetermined level; means for generating a magnetic field pulse bydischarging said first storage element into a second storage element viaa magnetic field inducing device; means for generating another magneticfield pulse of a different polarity and a different magnitude than thatof said previous pulse by discharging said second storage element intosaid first storage element via said magnetic field inducing device; saidgenerating means being arranged to be operated alternately to provide aseries of alternating polarity magnetic field pulses of decreasingmagnitude in said device.
 23. An apparatus for demagnetising a permanentmagnet by spiralling it around its hysteresis loop, said apparatuscomprising: means for generating an electromagnetic field; first andsecond charge storage means connected together and to said fieldgenerating means for transferring charge between each other via saidfield generating means; and control means for controlling the chargingand discharging thereof so as, in use of the apparatus, to cause saidfield generating means to generate an alternating polarity reducingmagnetic field.
 24. In or for an apparatus for altering the magneticstate of a permanent magnet by application thereto of a magnetic fieldof alternating polarity and decreasing strength, a control circuit forcontrolling the generation of said magnetic field by discharge of firstand second charge storage means through a magnetic field generatingmeans, said control circuit being adapted and arranged for transferringcharge between said first and second charge storage means by way of saidmagnetic field generating means so as to subject a permanent magnetwithin the magnetic field of said magnetic field generating means to asequence of alternating polarity magnetic impulses of progressivelydecreasing strength appropriate to the demagnetisation of the permanentmagnet.